In the production of bags from a synthetic resin foil web, apart from welding devices, which can be used to separate the bags from the web and simultaneously form lateral seams on the bag which are produced, frequently other devices must be provided along the path of the web to form various seams, produce various cutouts, incise openings, or to otherwise modify the shape or character of the bag which is ultimately to be separated from the web.
Typical among such devices are the devices for forming corner welds or gussets, devices for punching or incising ventilation holes and/or hanger holes, and the like. These additional devices are spaced apart by a distance equal to the increment of advance of the web or a multiple thereof and perform their respective operations in registry with the article to be produced at each location so that the openings and seams are all properly positioned in the finished bag when it is finally separated from the web. The increment of advance of the web is usually equal to the distance corresponding to a width of the bag or between successive separation seams. That distance is also referred to as a repetition length.
However, the mounting of a plurality of devices which perform independent operations on the web on a common carriage cannot solve the problem since each device must always be exactly positioned and for the most part, with the system described, only the one device which is in the immediate vicinity of the photocell may be positioned with sufficient accuracy. In the past, therefore, one had to be satisfied with a certain degree of imprecision with respect to the other devices of the bag-making line.